1. Field of the Invention
The present invention relates to a packaging material for flat electrochemical cell exhibiting stable sealing properties, insulation properties and moldability.
2. Description of the Related Art
A lithium ion battery is also called a lithium secondary battery and includes one containing an electrolyte in a liquid, gel or high polymer form and having a positive electrode substance and a negative electrode active substance, each of which is made of a high polymer. This lithium ion battery is a battery in which a charge and discharge reaction proceeds in such a manner that at the time of charge, a lithium atom (Li) in a lithium transition metal oxide which is a positive active substance becomes a lithium ion (Li+) and comes into carbon layers of a negative electrode (intercalation), whereas at the time of discharge, the lithium ion (Li+) comes out from the carbon layers (deintercalation), moves into a positive electrode and returns to an original lithium compound. The lithium ion battery has excellent advantages that not only it has a high output voltage and a high energy density as compared with a nickel-cadmium battery and a nickel-hydrogen battery, but it is free from a so-called memory effect that repetition of shallow discharge and recharge lowers an apparent discharge capacity.
Also, the lithium ion battery is configured of a positive electrode collector material/a positive electrode active substance layer/an electrolyte layer/a negative electrode active substance layer/a negative electrode collector material and an exterior body for packaging them. As a packaging material for forming the exterior body, a metallic can obtained by press working of a metal into a container in a cylindrical or rectangular parallelepiped form or the like has hitherto been used.
However, in the metallic can, a container external wall is rigid, and therefore, the shape of a battery itself is limited, and there is no degree of freedom in the shape due to the necessity that the hardware side must be designed in conformity with the battery. Accordingly, in recent years, there is a tendency that a multilayered film is used as the packaging material in place of the metallic can. This packaging material is configured of at least a base material layer, a metal foil and a thermally adhesive resin layer. An exterior body of a pouch type prepared by forming the packaging material in a bag-like state and storing a battery main body therein or an exterior body of an embossing type prepared by press working of the packaging material to form a recess and storing a battery main body in the recess is formed. For example, JP-A-2005-56729 proposes, as a packaging material for battery, a packaging material obtained by successively laminating a non-stretched polypropylene layer having a thickness of more than 10 μm and not more than 60 μm; an acid-modified polypropylene layer for bonding a metal foil and a thermally adhesive resin layer and having a thickness of from 1 to 5 μm; an aluminum foil layer having a thickness of from 10 to 100 μm, on the surface of which is formed a first chemical conversion coating layer having a coating amount of from 5 to 30 mg/m2; and a layer composed of a synthetic resin.
FIG. 12A is a perspective view of a related-art lithium ion battery 1 of a pouch type; and FIG. 12B is a diagrammatic exploded perspective view showing a related-art lithium ion battery of a pouch type. As illustrated in FIGS. 12A and 12B, in the lithium ion battery 1 of a pouch type, a lithium ion battery main body 2 is sealed and contained in an exterior body 10 formed in a bag-like state. Also, FIG. 13A is a perspective view of a related-art lithium ion battery 1 of an embossing type; and FIG. 13B is a diagrammatic exploded perspective view showing a related-art lithium ion battery of an embossing type. As illustrated in FIGS. 13A and 13B, in the lithium ion battery 1 of an embossing type, a lithium ion battery main body 2 is sealed and contained by using an exterior body 10 composed of a tray 10t and a sheet 10s each having an embossing part formed therein.
In all of these types, in sealing the lithium ion battery main body 2 by the exterior body 10, metal terminals 4 which are connected to a positive electrode and a negative electrode of the lithium ion battery main body 2, respectively are protruded to the outside of the exterior body 10, and the metal terminals 4 are sandwiched by the exterior body 10, followed by heat sealing to secure sealing properties. Here, the lithium ion battery main body 2 is configured of a cell (electricity storage part) including a positive electrode composed of a positive electrode active substance and a positive electrode collector body, a negative electrode composed of a negative electrode active substance and a negative electrode collector body and an electrolyte to be filled between the positive electrode and the negative electrode (all of which are not illustrated); and the metal terminals 4 which are communicated with the positive electrode and the negative electrode, respectively in the cell and which are protruded to the outside of the exterior body 10.
Also, in case of actually using the lithium ion battery 1, the impact resistance is weak only by the exterior body 10, and therefore, there is a possibility that a crack is generated due to a small scar. Accordingly, the lithium ion battery 1 is often contained in a plastic case and provided for use.
FIG. 14A is a diagrammatic perspective view showing a related-art lithium ion battery 1; and FIG. 14B is a diagrammatic perspective view showing a related-art lithium ion battery 1 contained in a plastic case 13 as illustrated by a dotted line.
Here, in the case where it is contemplated to miniaturize the lithium ion battery 1, a peripheral sealing part 10b of the exterior body of the lithium ion battery 1 must be folded and contained in the plastic case 13. FIG. 14C is a cross-sectional view of the lithium ion battery 1 contained in the plastic case 13 as seen from a direction shown by an arrow X in FIG. 14B.
However, in a folding part 10c which is a fold of an inner edge of the peripheral sealing part 10b, a thermally adhesive resin layer which is an innermost layer of the exterior body 10 is once melted at the time of heat sealing and then crystallized, and therefore, a crack is liable to be generated at the time of folding. Also, in the case where the generation of such a crack is caused, the electrolyte in the inside of the exterior body 10 comes into contact with a metal foil configuring the exterior body 10, and the metal foil is energized. Therefore, there is a problem that an output of the lithium ion battery is markedly lowered, whereby the function as a battery is lost.
Also, even in en embossing process of the exterior body 10 by means of press working, in the case where a metal with excellent extendibility, for example, aluminum is used as the metal foil configuring the exterior body 10, the stretching of the thermally adhesive resin layer as the innermost layer of the exterior body 10 does not catch up to the elongation of aluminum, resulting in possibilities of separation between aluminum and the thermally adhesive resin layer and generation of a crack in the thermally adhesive resin layer. Also, the packaging material for battery as disclosed in JP-A-2005-56729 did not satisfactorily solve the foregoing problems.
Also, besides the case of containing the lithium ion battery main body 2 in the exterior body 10, when a capacitor or an electric double layer capacitor is contained and sealed, the same problems are caused.